This invention relates generally to video signal transimission systems and specifically to a low power data transmission system for use in conjunction with a HDTV system.
The spectrum compatible high definition television (HDTV) system recently proposed by Zenith Electronics Corporation has the capability of transmitting television signals without objectionable interference with NTSC co-channels and adjacent channels (the so-called "Taboo" channels). This is accomplished in part by minimizing the power of the transmitted signals by removing the low frequency portion of the video signal, which requires high transmission power, developing a digital representation thereof and transmittig the digital data with the high frequency portion of the video signal. The digital data is transmitted during the vertical blanking interval (VBI) of the television signal. This approach, when correlated with "frame locking" to the co-channel or adjacent channel signal helps by "hiding" the data in the VBI of the other signal. Thus any interference caused will be much less visible.
Major considerations in transmitting digital data as part of a television signal include the bandwidth required for its transmission and the potential of interference into co-channels and adjacent channels. Therefore the data in the VBI is preferably transmitted at as low a power level as possible, consistent with good recovery, to maintain the noninterfering characteristics of the transmission system. Additionally, it is desirable to use data compression techniques to minimize the required transmission bandwidth for the data.